Wiring LED warning lights on automotive gauges

[drfrink24]

Ok, I know, this is an audio website. But, I think the principles are the same here. I know alot of you are immensely talented at figuring out electronic solutions. So, here is my question:

I have automotive aftermarket gauges, but, they are mechanical gauges w/senders, rather than the more expensive, computer controlled digital gauges. ($600 for three vs. $1100+ for 3)

They are pretty simple. They have light, neg/pos and ign, and then a sender hookup, which is basically 2 wires (in most cases).

I assume the sender is simply supplying a given voltage for a given pressure or heat. Maybe resistance, I'm not sure (as I have not tested them with my DMM yet).

All I want to do is have the sender wires pass thru a DIY device that allows me to set a yellow warning light, and a red danger light, based on input from the sender. The singnal needs to go onto the gauge without being changed, so that they still function normally.

This has to be simple stuff to do, assuming I can measure just how these senders are informing the gauge w/whats going on.

Any help would be greatly appreciated.

Any ideas?

[dnsey]

Search for comparator circuits. Easily implemented with op-amps, but you'll need to do a little measurement and calculation to design what you need.

[oPossum]

Many automotive sensors are a variable resistance. One wire sensors used to be quite common. They used a metal case for the ground. There are many possible problems with doing this, so two wire sensors are now most common. One wire will typically be ground. The gauge will supply some current to the sensor, so there will be a voltage on one of the sensor leads that you can monitor with a comparator circuit.

The LM324 would work fine for this. It is common, cheap, and runs off a single supply. The LM339 is another good choice. Both are quad chips. Use a regulated 8 or 9 volt supply for power and reference voltage. LM7808/LM7809 chips for that.